Chemical interesterification of a triglyceride fat aims at an exchange of the fatty acid residues of the glyceride moiety of the fat. After interesterification, on the resulting triglycerides the fatty acid residues have been exchanged by other residues. The fatty acid residues may originate from the same or from a different triglyceride molecules or they may come from free fatty acids which were present in the reaction mixture.
The exchange of fatty acid residues eventually results in a statistically random distribution of the fatty acid residues over the terminal and middle positions of the glyceride molecule. The obtained fat is said to have become fully randomised.
The chemical interesterification process needs a catalyst, which usually is an alkali metal hydroxide or an alkali metal alkanolate, such as sodium methanolate.
However, consumers increasingly prefer food and food ingredients which have not been exposed to chemicals during their preparation. Therefore a general need has arisen for non-chemical modification processes of triglyceride fats. Interesterification may also occur via enzymatic rearrangement. Such enzymatic process does not affect the naturalness of the fat.
Contrary to chemical interesterification which proceeds instantaneously, enzymatic rearrangement proceeds gradually, and therefore takes more time.
For enzymatic rearrangement (ER) a lipase enzyme is used as catalyst. Lipases used for ER comprise the microbial Mucor miehei lipase, Thermomyces lanuginosa lipase and Rhizopus oryzae (formerly Rhizopus delemar).
Generally, the lipases used in an ER process are sn-1 and sn-3 specific meaning that only the terminal fatty acid residues are effected.
In the course of the enzymatic reaction, some randomisation at the middle position may occur. However when this happens it is due to acylmigration (see Torres et al., JAOCS vol 79, no. 8 (2002) p 775-781, Torres et al JOACS, vol 79 no 7 (2002) p 655-661, and Zhang et al JAOCS vol 78, no. 1 (2001) p 57-64) which is a chemical side reaction which take place at long reaction times. The acylmigration is due to the presence of diacylglycerides which arise abundantly at long reactions times and in the presence of water.
This difference in randomisation results in triglyceride products with a triglyceride composition and with properties that are quite different from the fully randomised triglyceride fat resulting from chemical interesterification. Unfortunately, the extensive knowledge and experience acquired by using fully randomised chemically interesterified fats for manufacturing food products can not be used for enzymatically interesterified fats (Zhang et al JAOCS, Vol. 78, no 1 (2001) pp. 57-64).
Furthermore the middle position in a natural feedstock fat is usually is an unsaturated fatty acid, often oleic acid. Triglycerides of the type palmitic-oleic-palmitic may cause graininess in the fat blend. Because the middle position of the triglycerides in an enzymatic rearrangement reaction is hardly affected, triglycerides with a saturated middle position are barely present in enzymatically rearranged fats, unless already present in the starting material. This typical distribution of natural fatty acids over the triacyglycerides has some consequences. In the first place it is nutritionally beneficial to have an unsaturated fatty acid at the middle position, since the lipase activity in our digestive system delivers a 2-monoacylgyceride and 2 free fatty acids which are derived from the terminal triacylglycerides positions. This digestive effect is confirmed by Nielsen (Oils and fats international (vol 18, no 4 (2002)). He established that immobilised Lipozyme TL IM action is restricted to the 1 and 3 position on the triglyceride, leaving the middle position unaltered. However, this configuration of fatty acids over the glycerol backbone of the triacylglycerides also has a downside. With respect to food structuring functionality these triacylglycerides, with a unsaturated middle position, are less functional. This is due to their lower melting point compared to fully saturated triacylglycerides and their complicated crystallisation behaviour.
As explained before, rearrangement on the middle position may occur during enzymatic rearrangement, however in order to occur at a appreciable amount the enzymatic rearrangement has to proceed at equilibrium (100% conversion of sn1 and sn-3 position) or beyond. Chemical rearrangement proceeds instantly, meaning that instantly a complete randomisation is obtained. It is the nature of the enzymatic reaction that in the beginning of the reaction the conversion of sn-1 and sn3 position runs quickly, but that towards equilibrium the conversion rate proceeds more and more slowly (see FIG. 1). Consequently, to attain 100% conversion long enzyme contact times are needed.
The enzymatic re-arrangement process, even though strictly sn-1 and sn-3 specific, is always accompanied by a some change of the fatty acid distribution on the sn-2 position. This is due to the unavoidable chemical process of acyl-migration that occurs in partial fatty acid glycerides. Xu et al (Enzymatic Production of Structured lipids: Process reactions and Acyl migration, inform 11 (2000) p 1121-1131) reported that acylmigration can be attributed primarily to longer residence times. However, the related low flow through the packed bed reactor makes the process expensive for use on industrial scale. (Xu et al JAOCS, Vol. 79, no 6 (2002) pp. 561-565). Indeed, Torres et al. recommend short reaction times to reduce randomisation of the fatty acids residues (JOACS, vol 79, no 8 (2002) p 775-781).
Without wishing to be bound by theory the process of acylmigration is independent on the enzyme used, however it is due to the relatively slow rate of the process. Significant effect on the middle position only occur at very high conversion rates, often 100%, which relates to very long contact times. This is illustrated by FIG. 3.
The processes reported in the prior art typically refer to time and enzyme concentration combinations that relate to 100% conversion of the sn-1 and sn-3 position (equilibrium) and often in excess of the time needed to obtain 100% conversion on the terminal positions. As a logical consequence these reactions yield also a certain amount of randomisation of the middle position. However, these processes are economically not attractive, because of the long contact times needed to obtain a reasonable amount of sn-2 randomisation.
For example Berben et al in society of chemical industry (online 16 Feb. 2001) describe a process of enzymatic rearrangement wherein they have the reaction proceed until equilibrium and obtain a randomisation on the middle position of 18%.
WO96/14756 describes ER of fat blends using sn-1 and sn-3-specific SP392 as lipase catalyst. The process is characterised in that the rearrangement does not proceed beyond a conversion degree of the sn-1 and sn-3 position of 90% (but being at least 20%), which results in shorter reaction times. However, no randomisation at the sn-2 position is observed.
Some rare lipases including Candida cylindracae and Arthrobacter lipases are non-specific. An ER process using those lipases delivers a fat rearranged at all glyceride positions. However, those lipases either have been found to be not suited for use at an industrial scale and/or have not been approved for food manufacture.
The process described in EP 652289 uses a common sn-1 and sn-3-specific lipase. The rearrangement requires the presence of a substantial amount of at least 4 wt. % of diacylglycerides (also denoted as diglycerides) in the reaction mixture. The fat becomes rearranged at all three positions, but at the end it contains much diglycerides and other byproducts, all of which need to be removed by a subsequent purification process.
A cost effective ER process is needed which results in substantial rearrangement also at the middle position. Such a process would make a new range of naturally modified triglyceride fats economically available.
It is therefore an object of the present invention to provide an enzymatic rearrangement process wherein an appreciable amount of rearrangement on the middle position occurs. A further object of the invention is a process with a short reaction time. Another object of the invention is to provide an enzymatic rearrangement process without the deliberate use of diacylglycerides.
Surprisingly one or more of the above mentioned objects is obtained by using a catalyst with an activity exceeding 250 IUN (22 g/(g*h)) as measured at the onset of the process.